#include<bits/stdc++.h>
using namespace std;
#define int long long

int f[100][3];	// f[i][j]表示走j阶到第i层

signed main() {
	int n;
	cin >> n;

	f[1][1] = f[2][2] = 1;
	f[1][2] = f[2][1] = 0;
	for (int i = 3; i <= n; i++) {
		f[i][1] = f[i - 1][2];
		f[i][2] = f[i - 2][2] + f[i - 2][1];
	}
	cout << f[n][1] + f[n][2];
	return 0;
}
